When burning coal in the furnaces of steam generating units, one of the significant costs, both for original equipment and operation, is for dealing with the ash contained in the coal, particularly if a high ash-bearing coal is involved. In one type of present day coal-fired unit, the coal is pulverized to a flour-like consistency in a pulverizer and then carried in an airstream to the furnace where it is burned. Large pieces of mineral matter, such as tramp iron, which are too heavy to be picked up in the airstream, are discarded from the pulverizer. Also, a classifer located in the outlet from the pulverizing mill is set to separate a percentage (such as 10-20%) of the larger, heavier particles carried in the airstream and return them to the pulverizer for further grinding.
A large portion of the particles returned to the pulverizer for further grinding is ash, which is made up in large part by mineral matter, such as pyrites and quartz. Many of these particles never do get pulverized to a flour-like fineness, and thus they are separated out by the classifier and recirculated back to the pulverizer time after time. This greatly reduces the operating capacity of the pulverizer and also substantially increases the wear of the grinding apparatus, resulting in more frequent maintenance shutdowns. In large steam generating units, which utilize a number of pulverizing mills, the reduced capacity caused by the repeated recirculation of the "heavies" back to the pulverizer may require that an additional one or two mills be installed when the plant is first built, over that which would be required without this large amount of recirculation.
In spite of the fairly effective separation of the mineral matter from the coal in the classifier, some does still get carried over to the furnace. This might only represent a few percent (10% or less) of the entire amount of coal introduced to the furnace, but it contributes in a large sense to the maintenance and operating costs of the steam generator. The steam generator will have reduced generating capacity and availability because of the problems with furnace slag deposits. More soot blowers are required, and more frequent operation thereof is necessary when there is more than a minimal amount of ash in the coal being introduced into the furnace of a steam generator. Also, some of the mineral matter, in the form of pyritic sulfur, contributes to the SO.sub.2 problems in the combustion exhaust gases. From the above, it can be seen that if all but a minimal amount of the coarse ash particles could be economically removed from the coal leaving a pulverizing mill flowing to a steam generator, it would be highly desirable.